Contamination of jet fuels is often caused by surfactants that may come from the refining process and treatment, or from residue in pipelines or carriers as well as from cleaning and maintenance materials. The problems that are attributed to surfactants are principally related to their tendency to prevent the filter separator from performing its function, i.e., removing dirt and water from the fuel. The results of this are extensive. Water and dirt in an aircraft fuel system have well recognized dangers, but secondary effects are of equal importance. Bacteria can grow in the aircraft fuel system if water is present and the result can be corrosion of structural members and errors in the signals from fuel quantity gaging probes.
Treatment of fuel to remove surfactants is usually done with attapulgus clay. For best results with the clay treatment, it is essential to keep the fuel in contact with the clay for as long a time as possible. This is called residence time and is required because the fuel must have enough time to penetrate the clay particles where the surfactants and small dirt particles, in the sub-micronic range, can be adsorbed onto the microscopic surfaces of the crystals.
In the refinery, large towers are built to contain many tons of bulk clay in a percolation column. However, complications of handling bulk clay outside of a refinery have led the industry to use stacks of clay filter elements in a filter vessel when it is necessary to treat the fuel in field installations.
These conventional filter vessels use one or more of the replaceable filter elements stacked end to end on each of a series of outlet pipes. The liquid enters the filter vessel to pass through the filter elements radially to reach the outlet pipes. Flat plates, at the ends of each stack, press the filter elements together to seal the ends and the junctions between the elements.
Two types of clay filter elements are available. One is a bag element which contains a given quantity of clay inside an annular cloth bag. The other is a canister-type filter element in a rigid container, which contains the same clay but maintains its shape better. The canister-type element is several times more expensive than the bag element and it is more difficult to transport without damage, but it provides a more effective seal between the elements when they are stacked on top of one another in the standard filter vessels. The rigid ends can be equipped with gaskets to seal tightly and cut down on the by-passing of fluids, that may occur in bag-type elements because of the inevitable folds of the cloth in the adjacent ends of the bag elements. Since the rate of flow of the fluids through the clay elements is quite low, it is possible to have a significant amount of by-passing of fluids from relatively small creases or apertures between the bag elements.
Bag-type elements have the additional problems of being relatively amorphous, and having seams between the material of the inner, tubular portion, the outer tubular portion, and any end pieces that are provided. Such seams increase the possibility of poor sealing and by-passing of fluids, besides requiring additional sewing steps in the manufacture. The last sewing operation is particularly time consuming and must be made under difficult conditions, with the clay under the top of the filled element impeding the sewing operation and precluding a tight packing of the clay under the seams. Also the final seams cannot be internal, and raw edges must be exposed, which, again, increases the possibility of by-passing of fluids.
It is therefore an object of this invention to provide a bag-type filter element that is relatively easier, faster, and more economical to construct; that can be stacked in standard filter vessels; that is readily replaceable and disposable; and that virtually eliminates the by-passing caused by the folds and radial creasing that tends to develop at the ends of conventional bag-type filters.